The present invention relates to a control device of an auto-focus mechanism equipped in a camera and an improvement of focusing certainly and rapidly.
In the case that a zone for measuring a distance between an object and a camera is provided at a picture plane of the camera in an auto-focus mechanism in a type of a contrast method, a fixed and small zone is conventionally provided at a center portion of the picture plane. Usually, it is photographed a main object positioned at a center portion of the picture plane, so that a lens member is actuated in order to optimize a contrast ratio of a picture at the center portion of the picture plane.
However, when it takes a picture against the light, generally a contrast ratio is remarkably reduced at a center portion of the picture. It becaomes difficult to find an optimum contrast in the case of a conventional zone for measurng a distance and its focusing would become wrong.
In the case of an auto-focus mechanism in a type of a high frequency detecting method which is one of the contrast methods, an output of a picture image passes through an automatic gain control (AGC) circuit, a high frequency part of a signal of a picture image obtained from the automatic gain control circuit (hereinafter, it is referred as "a high frequency part") is picked up by passing a band pass filter, a picked-up high frequency part is detected in order to detect an evaluative value for focusig (hereinafter, it is referred as "an evaluative value"), and a proper focus position of a lens member where the evauative value becomes peak is determined. That is, when focusing is proper, a contrast ratio becomes maximum and a high frequency part also becomes maximum. In such cases, one kind of a band pass filter is utilized. Usually, a circuit for amplifing by a fixed gain value is provided at a wave detecter circuit in order to incerase a sensitivity for picking up the high frequency part. In addition, it is judged whether a peak is over when the evaluative value is going down after the evaluative value is going up. However, a noise is usually included in the evaluative value. Unless the evaluative value changed more than a noise level, it cannot be found that the evaluative value is actually increased or decreased. Therfore, in a conventional method, it is judged whether a displacement of an evaluative value is greater or less than a fixed reference value.
However, in a conventional auto-focus mechanism, there is only one band pass filter. In the case of a narrow band pass filter, if a focus lens is initially positioned far from a proper focus point, a filter output is little changed although the focus lens is moved frontwards or backwards. Since a high frequency part shifted to a relatively low frequency part and an amplitude of the signal is still small. Accordingly, the evaluative value is little changed initially so that it is very difficult to detect whether a frontward direction or a backward direction is a proper direction toward a focus point. It needs much time to detect a peak of the evaluative value certainly. In the case of a broad band pass filter, a displacement of the evaluative value is changed slightly near the peak, so that it is difficult to detect a real peak certainly.
The above described relation between focusing and an amount of a high frequency part comes into existence in the case that a brightness of an object is constant. If a brightness of an object is changed while a focus lens is moving for detecting a peak of an evaluative value, a wrong evaluative value is recognized as a peak. Since, a high frequency part is changed in accordance with a displacement of a brightness.
In the case that an object is dark. an iris is further opened and a gain value of AGC circuit becomes high. The high gain value of AGC is added to a fixed gain value of an amplifier circuit. Then, a gain value in a whole system becomes very high and a noise component is also increased. If the noise component is large, it is difficult to detect a change of the evaluative value although the evaluative value is increased or decreased a little. A peak of an evaluative value is unclear so that it cannot detect a proper focus position certainly. Further, a noise level is changed in accordabce with an opening degree of an iris, a gain value of AGC citcuit and a gain value of an amplifier in a wave detector. It is a problem described as follows, if a reference value for detecting a displacement of an evaluative value is fixed.
(1) In the case of a fixed reference value as a relatively low level, when an iris is relatively closed, a gain of AGC is relatively high or a fixed gain of an amplifier is relatively high, a noise is increased in the evaluative value. A judgement of a peak of the evaluative value becomes unclear due to the noise. As a result, the judgement are sometimes wrong.
(2) On the contrary, in the case of a fixed reference value as a relatively high level, when an iris is relatively opened, a gain value of AGC is relatively low or a fixed gain of an amplifier is relatively low, a peak of the evaluative value cannot be judged until the evaluative value is much decreased, although a noise is relatively less. That is, the peak cannot be deteced until a focus lens runs over a proper focus position some distance. A detection of the peak is delayed.
On the other hand, recently, there is a video camera in a rear focus lens type, in which a focus lens is provided behind a zooming lens. In the rear focus lens type, a proper focus position of a focus lens is changed depending on a focusing distance of a zooming lens. The focus position is changed as a curved line.
Conventionally, it is difficult to successively change a position of a focus lens in accordance with a focus distance of a zooming lens.
A purpose of the present invention is to resolve the above descrived problems, particularly, the first purpose of the present invention is to provide an auto focus control device in an auto focus mechanism in a type of a contrast method by detecting a high frequency part to judge a proper direction of a focus lens toward a focus position and a peak of an evaluative value.
The second purpose of the present invention is to provide an auto focus control device in an auto focus mechanism in a type of a contrast method by detecting a high frequency part to prevent from wrong focusing by detecting an evaluative value at a right focusing timming and positioning a focus lens at a point where the evaluative value becomes maximum.
The third purpose of the invention is to provide an auto focus control device in an auto focus mechanism in a type of a contrast method by detecting a high frequency part to focus a dark object correctly in a case that an evaluative value is detected and a focus lens is positioned at a point where the evaluative value becomes maximum.
The fourth purpose of the present invention is to provide an auto focus control device in an auto focus mechanism in a type of a contrast method by detecting a high frequency part to judge a peak of an evaluative value correctly without mistaking in the case that a focus lens is positioned at a point where the evaluative value becomes peak.